The present invention relates to a lens inspection system for inspecting a lens used for an optical disc drive or the like for measuring and examining a wave front of a light beam emitted by the lens.
In an optical data recording/reproducing device using an optical disc, if an objective lens is damaged and/or the objective lens is not in place with respect to the data recording surface of the optical disc, coma, astigmatism or some other wave front aberrations may occur. In such a situation, a beam spot on the optical disc may not be sufficiently small on the data recording surface of the optical disc, which prevents recording/reproducing of data at high density on the optical disc. Therefore, when the optical disc is manufactured, the wave front of the light emerged from the objective lens is measured using an interferometer in order to examine a performance of such a device.
Recently, in order to increase the recording density of the optical disc, optical data recording/reproducing technology using a high-NA (numerical aperture) objective lens has been developed. In order to measure the entire wave front of light emerged from such a high-NA objective lens, the interferometer should also be provided with an objective lens whose NA is higher than the NA of the objective lens of the optical disc drive.
However, the high-NA objective lens of the interferometer is required to have a very high surface accuracy, and the wave front aberration of such a lens should be suppressed within a predetermined allowable level. Such a lens is very difficult to manufacture, and would be relatively expensive. Further to the above, a precise structure of the interferometer is required in accordance with the accuracy of the high-NA objective lens.
It is therefore an object of the invention to provide an improved lens inspection system capable of measuring a wave front of light emerged from a high-NA lens of an optical disc drive without using a high-NA objective lens for the interferometer.
For the above object, according to the invention, there is provided a lens inspection system for evaluating an objective optical system of an optical data recording/reproducing device by measuring a wave front of a light beam emerged from the objective optical system, provided with:
a laser source, a laser beam emitted by the laser source being incident on the objective optical system of the optical data recording/reproducing device;
an interferometer for measuring the wave front, a numerical aperture of an objective lens of the interferometer being smaller than a numerical aperture of the objective optical system of the optical data recording/reproducing device; and
a conversion lens which receives the light beam emerged from the objective optical system of the optical data recording/reproducing device and emerges a light beam to the objective lens of the interferometer such that all the light beam emerged from the conversion lens enters the objective lens of the interferometer.
As above, by the conversion lens, all the light emerged from the objective optical system of the optical data recording/reproducing device can be received by the interferometer even though the numerical aperture of the objective lens of the interferometer is relatively small.
Optionally, the conversion lens may have a first surface including a spherical surface, and a second surface that is a flat surface perpendicular to a central axis of the light beam emerged from the objective optical system of the optical data recording/reproducing device. The conversion lens may be arranged such that the first surface faces the objective lens of the interferometer, and the second surface faces the objective optical system of the optical data recording/reproducing device, with the central axis of the light beam passing a spherical center of the first surface.
Optionally, a parallel plate is inserted between the objective optical system of the optical data recording/reproducing device and the conversion lens. Preferably, the parallel plate has a thickness and refractive index that are similar to those of a transparent base plate of an optical disc. The parallel plate enables a precise measurement of the wave front.
In this case, the conversion lens and the parallel plate may be apart from each other.
In particular, the parallel plate may be a dummy disc that is mounted on a spindle of a data recording/reproducing device.
Optionally, the second surface of the conversion lens may be located at a level where the data recording surface of the optical disc is located when data is recorded/reproduced, and at least a portion of the dummy disc facing the conversion lens is shifted toward the objective optical system side.
Alternatively, the conversion lens contacts the parallel plate. In this case, the conversion lens and the parallel plate could be integrally formed.
Further optionally, the conversion lens is made contact with a surface of the objective optical system of the optical data recording/reproducing device. In this case, a numerical aperture of the objective optical system of the optical data recording/reproducing device could be greater than one.
Still optionally, the conversion lens may be a hemispherical lens. In this case, a point where the light beam emerged from the objective optical system of the optical data recording/reproducing system may converge substantially coincides with the spherical center of the hemispherical lens.
According to another aspect of the invention, there is provided a lens inspection system for measuring a wave front of a light beam emerged by an objective optical system of an optical data recording/reproducing device, which is provided with:
an interferometer for measuring the wave front, a numerical aperture of an objective optical system of the interferometer being smaller than a numerical aperture of the objective optical system of the optical data recording/reproducing device; and
a conversion lens that receives the light beam emerged from the objective optical system of the optical data recording/reproducing device and emerges the light beam, all of which enters the objective optical system of the interferometer.
Optionally, the conversion lens is a hemispherical lens, a point where the light beam emerged from the objective optical system of the optical data recording/reproducing system converges substantially coincides with the spherical center of the hemispherical lens.